What is the force that oftentimes makes a moving object change its speed or eventually completely stop

If a rock is thrown upward on the planet mars with a velocity of 10 mys, its height in meters t seconds later is given by y − 10t 2 1.86t 2 . (a) find the average velocity over the given time intervals: (i) [1, 2] (ii) [1, 1.5] (iii) [1, 1.1] (iv) [1, 1.01] (v) [1, 1.001] (b) estimate the instantaneous velocity when t − 1.